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Highlights and News at MBI
Earlier Highlights are found in the archive
28th May 2015: Are rogue waves predictable?
A comparative analysis of rogue waves in different physical systems comes to the surprising conclusion that these rare events are not completely unpredictable. ... more.
 
10th April 2015: Rubicon fellowship for Geert Reitsma
The Netherlands Organisation for Scientific Research (NWO) rewards MBI researcher Geert Reitsma a prestigious Rubicon Grant for his proposal "Filming biomolecules in action". With this grant he will continue his career at the MBI.. ... more.
 
26th March 2015: Fast Gold
A new laser plasma acceleration scheme for heavy ions is found, leading to a significant boost of their
kinetic energy by Coulomb explosion. ... more.
 
24th March 2015: Classical or not? Physics of nanoplasmas
The interaction of an intense laser pulse with a nanometer-scale particle results in the generation of an expanding nanoplasma. In the past, nanoplasma dynamics were typically described by classical phenomena, like the thermal emission of electrons. In contrast, a new study on the interaction of intense near-infrared (NIR) laser pulses with molecular oxygen clusters now demonstrates that phenomena, which can only be described quantummechanically, play an important role. ... more.
 
11th February 2015: New Insights into the photophysics of the DNA base thymin
DNA stores our genetic code. Solar UV radiation has sufficiently high energy to basically break bonds of the DNA and thus cause DNA damage. Although DNA (e. g. in our skin cells) is exposed to intense UV light irradiation, DNA proves to be surprising photostable. ... more.
 
10th February 2015: Nonlinear resonance disaster in the light of ultrashort pulses
Ultrashort light pulses from modern lasers enable temporal resolution of even the fastest processes in molecules or solid-state materials. ... more.
 

 
More detailed Information:

 


Are rogue waves predictable?


28. May 2015

A comparative analysis of rogue waves in different physical systems comes to the surprising conclusion that these rare events are not completely unpredictable.

 

Monsterwelle
Caption: Detail of "The Great Wave off Kanagawa" by Katsushika Hokusai, which has been frequently discussed to depict an ocean rogue wave.
Metereological events often prove to be rather unpredictable, i.e., the "storm of the century" may well prove to be surpassed by yet another storm just in the subsequent year. From an insurance point of view, resulting damage often proves to be be well beyond any statistical prediction. Such phenomena generally underlie extreme value statistics, featuring a prevalent appearance of extreme events and contrasting long-term observations of rather normal events in the respective system. Rogue waves, also known as freak waves, are yet another example for such dynamics. While being extremely rare events, their appearance may cause considerable damage to the hull of ships.

 

The precise origin of rogue waves is still disputed. Moreover, it is unclear whether rogue waves can be predicted. Maybe, it is possible to issue a last-instant warning from observations of recorded wave heights? Do characteristic patterns exist that herald the impact of such a rogue wave? Unfortunately, there are only a few recordings of such ocean freak waves. Consequently, it may well take many more decades to answer those questions based on oceanic observations only. Nevertheless equivalent physical systems exist, which allow an exploration of this aspect at a substantially more solid statistical basis.

This is the point where the work of Simon Birkholz and coworkers sets in. Based on a statistical analysis of data in three different physical systems, the group conducted a detailed analysis on the predictability and determinism in the respective system. This analysis included original data of the famous New Year’s Wave, which hit the Draupner platform on January 1, 1995 as well as results of the Jalali group at the University of California at Los Angeles (UCLA), and finally data in a multifilament scenario measured at the Max-Born-Institut in Berlin. ┬áIn the multifilament system, one can directly observe the rogue waves as short light flashes in the intensity profile. The wave height of the ocean system corresponds to light intensity in the optical systems.

The surprising result of this comparative analysis is that rogue events appear to be very much predictable in certain system, yet are completely stochastic and therefore unpredictable in others. In other words, rogue wave statistics does not enable any conclusion on predictability and determinism in the system. It is simply not true that rogue events per se appear out of nowhere and disappear without a trace. Ocean waves play a particular role here. Other than previously assumed, they are not completely stochastic. Therefore it is not true that they “appear out of nowhere and leave without a trace”, which has often been claimed to be a characteristic feature of ocean rogue waves. Nevertheless, practical predictions are still far away and may only enable a last-second warning of these “monsters of the deep”.

d Fig.: Snapshot of a rogue event in multifilament dynamics recorded in
a xenon cell at 60 times the critical power for filamentation.
The optical fluence is plotted as a function of position on the optical detector.
Interview

only in German available

Logo - Das Wissenschaftsmagazin: Monsterwellen und ihre Vorhersehbarkeit von: Michael Kurth
gesendet am 19.06.2015 auf Norddeutschen Rundfunk, NDR Info (Ausschnitt Dauer: Minute ca. 7:33 bis 14:07)

   

Original publication: Physical Review Letters 114, 213901 Predictability of Rogue Events

Simon Birkholz, Carsten Brée, Ayhan Demircan, and Günter Steinmeyer (Editor’s suggestion)

Contact

S. Birkholz
Dr. G. Steinmeyer

 
     
 


Rubicon fellowship for Geert Reitsma

10. April 2015

The Netherlands Organisation for Scientific Research (NWO) rewards MBI researcher Geert Reitsma a prestigious Rubicon Grant for his proposal "Filming biomolecules in action". With this grant he will continue his career at the MBI.

Many processes of life rely on ultrafast movements of complex molecules. One example for an ultrafast movement happens right now while you are reading these lines: When retinal in our eye absorbs light, the molecular structure changes extremely fast. This change ultimately induces an electrical signal transduced to the brain with the result that we see the light. The general sequence of this process is very well understood. However, the underlying electronic and nuclear rearrangement of the molecule is not adequately understood. The ideal way to get this understanding is filming such a process. As the process is very fast, the frame rate needs to be very high to capture individual rearrangements. One prerequisite to obtain this frame rate is very advanced ultrafast laser technology. Here, at the MBI, Reitsma will have access to the most advanced laser systems allowing him to produce these movies of biomolecules in action.

Dr. Reitsma did his PhD research in the Quantum Interactions and Structural Dynamics group at the Zernike Institute for Advanced Materials in Groningen (NL). He received his PhD from the University of Groningen on Dec. 1, 2014. The Netherlands Organisation for Scientific Research (NWO) is the most important science-funding body in the Netherlands and aims to ensure quality and innovation in science. Facilitated by NWO's Rubicon programme, 60 PhD graduates per year will be conducting research at top foreign institutes. Through Rubicon, NWO gives talented young scientists the opportunity to gain international research experience as a stepping stone to a scientific career.

More information: NWO's Rubicon programme

http://www.nwo.nl/en/news-and-events/news/2015/

Contact

Dr. Geert Reitsma

 

 
     
 


Fast Gold

26. March 2015

A new laser plasma acceleration scheme for heavy ions is found, leading to a significant boost of their kinetic energy by Coulomb explosion.

We are all made from stardust - this phrase contains a lot of unknown but exciting physics, which the poem writer probably did not want to tell. Within this context the top ten ranking of open problems in physics lists the question related to the synthesis of the heavy elements - stardust. To gain a deeper inside, from an experimental point of view, heavy ions with very high velocities are needed. The collision of heavy particles at high kinetic energy enables the study of the resulting reaction products. Beyond basic research, beams or pulsed beams of heavy ions are in the focus of applications in solid state physics as well as in bio-medical areas.

Such beams are produced with particle accelerators which belong to the biggest and most complex machines for research today. In order to make things more compact there is a strong motivation for searching new concepts and technologies in particle acceleration. One approach is acceleration by plasma created by laser pulses at relativistic intensities. The term relativistic indicates here that the intense light field causes an electron motion close to the speed of light and hence relativistic effects determine the properties of the laser-plasma interaction. Enormous fields with a strength of the order of megavolts per micrometer can be created in a well-defined direction leading to fast (some ten percent of light velocity) light and heavy ions. Due to the extreme acceleration these ion beams have some striking characteristics which are studied and applied in experiments.

The challenge of heavy ion acceleration results directly from basic principles: Ions are accelerated proportional to their charge to mass ratio Z/A leading to higher velocity for lighter elements. Laser plasma acceleration of really heavy ions e.g. gold, had been quite inefficient for this reason as very high ionization degrees are difficult to obtain. This limitation is overcome in our experiments with freestanding gold foils of nanometer thickness: which provide an so far unexpected high degree of ionization and specific distribution of the heavy material (Z>40+ for Gold ions). Compared to former experiments we achieved kinetic energies of the gold ions with 1 MeV per nucleon with an order of magnitude less laser energy.

Common laser plasma driven acceleration models assume an averaged degree of ionization, which follows with a fixed, spatially uniform electron density. From our theoretical considerations and simulations we recognized, that the obtained high kinetic energies of the heavy ions are not consistent with an average ionization. Instead, we found for the ultra-thin targets a spatially varying degree of the ionization, with the highest degree of ionization at the target boundaries (found by simulation - cf. Fig.) - where it greatly enhances the electrical field and thus the force driving the ion acceleration.

Extrapolation of our results envisions the study of nuclear processes if heavy ions with adequate kinetic energies will be produced by using femtosecond lasers with 100 Joule pulse energy.

Original publication: Physical Review Letters

Full citation:

J. Braenzel, A.A. Andreev, K. Platonov, M. Klingsporn, L. Ehrentraut, W. Sandner, M. Schnuerer, "Coulomb-Driven Energy Boost of Heavy Ions for Laser-Plasma Acceleration", Physical Review Letters 114, 124801 (2015)

doi: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.124801

Contact

Julia Braenzel

Dr. Matthias Schnuerer


Fig. 1:
Laser (1.3J @ 35 fs) irradiated 14 nm thick gold foils. The picture shows dependence of maximal ion energy on its ionization energy - as the experiment delivered (pink squares). We found good agreement with the 2D -PIC simulation - blue squares and with the prediction of our new theoretical model -blue line in contrast to the established, old model - black line

 

Fig. 1 (click to enlarge)  
 
     
 


Classical or not? Physics of nanoplasmas


24. March 2015

The interaction of an intense laser pulse with a nanometer-scale particle results in the generation of an expanding nanoplasma. In the past, nanoplasma dynamics were typically described by classical phenomena, like the thermal emission of electrons. In contrast, a new study on the interaction of intense near-infrared (NIR) laser pulses with molecular oxygen clusters now demonstrates that phenomena, which can only be described quantummechanically, play an important role. For the first time, evidence of efficient formation of autoionizing states in nanoplasmas is found. Autoionization of so called superexcited states of atomic oxygen is directly observed on a nanosecond time scale, whereas indirect signatures are visible for decay processes occurring on shorter time scales. Autoionization is found to take place in various systems and is expected to be important also in the interaction of finite systems with intense extreme-ultraviolet (XUV) and X-ray pulses from novel free-electron laser sources.

Following the interaction of intense NIR laser pulses with clusters, the recorded electron spectra typically show a smooth distribution. In the past, the absence of discrete state signatures in these spectra led to the conclusion that the dynamics of charged particles during the cluster expansion can be well described by fully classical behavior. As a consequence, simulations that model the interaction of intense lasers with clusters, nanoparticles or large molecules, often make use of quasiclassical approaches. With the advent of novel laser sources and time-resolved techniques during the last year, this picture began to falter. Recently, extensive formation of excited atoms in nanoplasmas driven by electron-ion recombination processes was reported. When an atom with 2 electrons in excited states is formed, it may decay via an electron correlation effect, where one electron is released into the continuum, while the second electron relaxes to a lower bound state. However, since the electrons emitted via such autoionization processes exchange kinetic energy with the cluster environment, they had not been observed in experiments so far.

In a collaboration led by scientists from the Max-Born-Institut, the first evidence of autoionization following intense NIR laser-cluster interactions is now reported. In the current issues of Physical Review Letters [114, 123002 (2015)] Bernd Sch├╝tte, Marc Vrakking and Arnaud Rouz├ęe, and their colleagues Jan Lahl, Tim Oelze and Maria Krikunova from the TU Berlin present results obtained from oxygen clusters. This system was chosen, because oxygen atoms have previously been shown to exhibit long-lived autoionizing states. In the present study, clear peaks were observed in the electron spectrum from oxygen clusters ionized by intense NIR pulses (Fig. 1). These peaks could be assigned to well-known autoionizing states, and it was shown that they decay on a nanosecond time scale, when the cluster has already significantly expanded. Therefore, the influence of the environment on the electrons emitted via autoionization was negligible. The observed autoionization contributions were found to be very sensitive on the intensity of the NIR laser pulse. At higher intensities, the autoionization peaks were blurred out, but still visible. These results indicate that autoionization plays an important role in many experiments that study the interaction of intense laser pulses with nanometer-scale systems, even when these processes cannot be directly observed in the electron spectrum. Previously, it was demonstrated that the observed nanoplasma dynamics following intense XUV and NIR ionization of clusters are similar, and therefore, the current results are expected to be highly relevant as well for experiments at novel free-electron lasers. The experimental findings of autoionization are also important for improving theoretical models of nanoplasmas in the future in order to gain a better understanding of the underlying microscopic processes.

The presented results demonstrate that a description of nanoplasma dynamics by classical approaches is insufficient. Quantum phenomena like autoionization play an important role during the expansion of clusters following the interaction with intense light pulses.

Originalpublication: Physical Review Letters

Full citation:

Bernd Schütte, Jan Lahl, Tim Oelze, Maria Krikunova, Marc J. J. Vrakking and Arnaud Rouzée, "Efficient autoionization following intense laser-cluster interactions", Physical Review Letters 114, 123002 (2015)

doi: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.123002

Contact:

Dr. Bernd Schütte

Prof. Marc J. J. Vrakking

Dr. Arnaud Rouzée

schuette2015

Fig. 1: (a) Two-dimensional electron momentum map emitted from O2 molecules, showing an anisotropic distribution of electrons peaked along (vertical)  the NIR laser polarization. (b) In the corresponding kinetic energy spectrum, the observed peaks are attributed to above-threshold ionization and Freeman resonances. (c) The electron momentum map from O2 clusters with an average size of 2400 molecules exhibits a much more isotropic behavior. (d) In the kinetic energy spectrum, three clear peaks emerge that are assigned to autoionization of superexcited atomic states.

 

Fig. 1  
 
     
 


New Insights into the photophysics of the DNA base thymine


11th February 2015

DNA stores our genetic code. Solar UV radiation has sufficiently high energy to basically break bonds of the DNA and thus cause DNA damage. Although DNA (e. g. in our skin cells) is exposed to intense UV light irradiation, DNA proves to be surprising photostable. It is well established that this is due to efficient mechanisms that convert electronic energy into other forms of energy, in particular heat. An important role is played by so-called conical intersections between electronic excited potential energy surfaces and the ground state potential energy surface. These conical intersections are associated with structural changes of the molecules. The exact pathways back into the electronic ground state however are topic of intense research.

Although DNA is a macro molecule with billions of atoms (in case of human DNA), it can still be divided into only a few structural (and functional) elements: four DNA bases, a sugar moiety and a phosphate group. The absorption of UV light exclusively takes place in the DNA bases. For this reason it is a common scientific approach to investigate the UV response of DNA bases, first.

A team of scientists from MBI and universities of Hokkaido and Hirosaki in Japan have for the first time investigated the DNA base thymine in aqueous solution by time-resolved photoelectron spectroscopy and questioned existing ideas about the excited-state relaxation process in this base. So far was supposed that a significant fraction of the excited-state population remains in a dark nπ* state instead of immediately returning to the ground state via a conical intersection. This dark state cannot be observed by optical spectroscopy (e. g. transient absorption or fluorescence upconversion), directly. Corresponding limitations however do not exist for photoelectron spectroscopy.

By combining experiment and simulation, for the first time two different relaxation pathways were identified. Both pathes evolve in the first excited state (ππ*). The faster reaction path is associated with a twist of the aromatic ring and leads to repopulation of the electronic ground state within 100 fs. The second path involves an out-of-plane motion of the carbonyl group, and the molecule returns to the ground state within 400 fs. The scientists did not find any indication for an important role of the second excited nπ* state and conclude that this state is not involved in the relaxation process.

Original publication:
Franziska Buchner, Akira Nakayama, Shohei Yamazaki, Hans-Hermann Ritze, Andrea Lübcke
Excited-State relaxation of hydrated thymine and thymidine measured by liquid-jet photoelectron spectroscopy: experiment and simulation, JACS,
JACS, DOI: 10.1021/ja511108u

folgt in K├╝rze

Fig. 1: After UV excitation thymine evolves on the (ππ*) excited state surface along two different reaction coordinates. The first involves a twisting of the aromatic ring, the second an out-of-plane motion of the carbonyl group. In contrast to existing ideas, the nπ* state does not seem to be involved in the relaxation process..

Fig. 1 (click to enlarge)


Contact:
Dr. Andrea Lübcke Tel: 030 6392 1207



 
     
 

Nonlinear resonance disaster in the light of ultrashort pulses


10th February 2015

Ultrashort light pulses from modern lasers enable temporal resolution of even the fastest processes in molecules or solid-state materials. For example, chemical reactions can, in principle, be traced down to the 10-fs time scale (1 femtosecond (fs) = 10-15 s). Ten femtoseconds correspond to a few oscillation cycles of the light field itself. Nevertheless, there is a class of optical processes that does not exhibit any measurable delay relative to the ultrafast light oscillation and which has been termed “instantaneous”. This class of processes includes nonlinear optical harmonic generation at multiple frequencies of the input field. This process is commonly used to generate the green light of laser pointers from invisible infrared light. These processes are normally used far away from a resonance to avoid losses.

In a collaborative effort, researchers of the Max-Born-Institut, the Weierstraß-Institut as well as the Leibniz-Universität Hannover now experimentally demonstrated for the first time that conditions exist where optical harmonic generation becomes non-instantaneous. Analyzing third-harmonic generation in titanium dioxide thin films, a lifetime of 8 fs was found, i.e., non-instantaneous behavior. Nevertheless, this process still qualifies as one of the fastest processes ever resolved with femtosecond spectroscopy.

Detailed theoretical modeling of these surprising findings indicates that this non-instantaneous response may only occur if there is a resonance of the third harmonic in the optical material. In turn, the generated material response persists to oscillate several cycles after the excitation has already ceased. Concomitantly, third-harmonic radiation is emitted. The process therefore appears like an atomic “resonance disaster”. Similar to mechanical oscillators, this atomic system therefore shows a non-instantaneous behavior.

These findings have important consequences for femtosecond measurement techniques and possibly also for ultrashort-pulse generation. These methods have always relied on an instantaneous nature of harmonic generation and related effects. Similar to soldiers who avoid marching in step on a suspension bridge, one therefore also has to carefully avoid optical resonances when measuring extremely short laser pulses.

Original publication:
Michael Hofmann, Janne Hyyti, Simon Birkholz, Martin Bock, Susanta K. Das, Rüdiger Grunwald, Mathias Hoffmann, Tamas Nagy, Ayhan Demircan, Marco Jupé, Detlev Ristau, Uwe Morgner, Carsten Brée, Michael Woerner, Thomas Elsaesser, Guenter Steinmeyer
Noninstantaneous polarization dynamics in dielectric media
OPTICA doi.org/10.1364/OPTICA.2.000151

 

ResponseJPG

Figure 1: Reaction of SiO2 and TiO2 to a short pulsed light field. In SiO2 the displacement of electron shell follows the exciting electric field. Immediately after the end of the pulse, this oscillation ceases, too. In contrast, in TiO2, an oscillation build-up is observed at the third harmonic of the exciting field. This oscillation continues beyond the end of the pulse. Insets show pictures of crystalline modifications for both optical materials (Photographs by Didier Descouens, CC BY 3.0 and Rob Lavinsky, CC-BY-SA-3.0).

Fig. 1 (click to enlarge)

visualize4

Movie: Reaction of SiO2 and TiO2 to a short pulsed light field. The electric field is visualized by the central arrow. The resulting displacement of the electron shell is shown in a simple atomic picture for both materials. Third-harmonic emission is indicated by a blue color of the shell. In SiO2, both the resulting oscillation as well as the harmonic emission immediately cease after the end of the exciting pulse. In contrast, TiO2 exhibits a resonant build-up of the third-harmonic oscillation, which persists beyond the duration of the exciting pulse.

Fig. 2 (click for animation - AVI-file)  


Contact
Dr. Günter Steinmeyer Tel: 030 6392 1440

 
     



Earlier Highlights and News can be found in the Archive. ... here